1. Field of the Invention
The starting point of the invention is a fiber-optic transmission sensor with modulator according to the precharacterizing clause of patent claim 1.
2. Discussion of Background
With the precharacterizing clause of patent claim 1, the invention refers to a prior art as is disclosed by Roland Stierlin, Faseroptische Sensoren [Fiber-optic sensors], Bulletin SEV/VSE 82 ( 1991), pp. 21-29. In the Sagnac sensor type described there in conjunction with FIG. 8, linearly polarized laser light is fed in parallel in 2 HB (high-birefringence) optical fibers to the sensor head and from there, in mutually opposing directions, via 2 .lambda./4-plates into the sensor fiber. The two .lambda./4-plates are at 45.degree. to the direction of the linear polarization in the respective HB fiber, so that right-circularly polarized light reaches the sensor fiber from both sides. The sensor fiber is a twisted LB (low-birefringence) glass fiber. After the passage through the respective second .lambda./4-plate, the light is once more converted back into the linear polarization condition. The polarization direction of the emergent light is, in so doing, parallel to that of the entry light. The light is fed back via the two HB glass fibers and is combined in a transmitting-evaluating unit in a first brancher and is coupled out from the input light path via an input polarizer in a second brancher. Using this Sagnac current sensor, the phase difference of the two light beams is measured by interference formation. A phase modulator, by means of which both light beams are phase-modulated, is fitted in an HB glass fiber, for example by sticking a wound portion of the HB glass fiber firmly on a piezoelectric element. By means of the propagation time difference which is produced by the right and left circulating light beam not passing the modulator at the same time, the phase of the two light beams is modulated in a different cycle. An analysis of the periodic change of the interference signal produces a DC and an AC current component of the modulation frequency, which components are evaluated by the formation of a ratio, as well as higher harmonics. The length of the sensor fiber is matched to the intensity and the frequency of the phase modulation. One of the 2 independent light beams is used as a reference for the other.
The signal evaluation in this type of sensor is relatively costly.